This disclosure relates generally to combined cycle fuel cell systems, and more particularly to high-efficiency solid-oxide fuel cell (SOFC) systems that achieve higher fuel cell conversion efficiencies than that achievable using conventional combined cycle systems.
Fuel cells are electrochemical energy conversion devices that have demonstrated a potential for relatively high efficiency and low pollution in power generation. A fuel cell generally provides a direct current (dc) which may be converted to alternating current (ac) via for example, an inverter. The dc or ac voltage can be used to power motors, lights, communication equipment and any number of electrical devices and systems. Fuel cells may operate in stationary, semi-stationary, or portable applications. Certain fuel cells, such as solid oxide fuel cells (SOFCs), may operate in large-scale power systems that provide electricity to satisfy industrial and municipal needs. Others may be useful for smaller portable applications such as for example, powering cars.
A fuel cell produces electricity by electrochemically combining a fuel and an oxidant across an ionic conducting layer. This ionic conducting layer, also labeled the electrolyte of the fuel cell, may be a liquid or solid. Common types of fuel cells include phosphoric acid (PAFC), molten carbonate (MCFC), proton exchange membrane (PEMFC), and solid oxide (SOFC), all generally named after their electrolytes. In practice, fuel cells are typically amassed in electrical series in an assembly of fuel cells to produce power at useful voltages or currents.
In general, components of a fuel cell include the electrolyte and two electrodes. The reactions that produce electricity generally take place at the electrodes where a catalyst is typically disposed to speed the reactions. The electrodes may be constructed as channels, porous layers, and the like, to increase the surface area for the chemical reactions to occur. The electrolyte carries electrically charged particles from one electrode to the other and is otherwise substantially impermeable to both fuel and oxidant.
Typically, the fuel cell converts hydrogen (fuel) and oxygen (oxidant) into water (byproduct) to produce electricity. The byproduct water may exit the fuel cell as steam in high-temperature operations. This discharged steam (and other hot exhaust components) may be utilized in turbines and other applications to generate additional electricity or power, providing increased efficiency of power generation. If air is employed as the oxidant, the nitrogen in the air is substantially inert and typically passes through the fuel cell. Hydrogen fuel may be provided via local reforming (e.g., on-site steam reforming) or remote reforming of carbon-based feedstocks, such as reforming of the more readily available natural gas and other hydrocarbon fuels and feedstocks. Examples of hydrocarbon fuels include, but are not limited to, natural gas, methane, ethane, propane, methanol, and other hydrocarbons.
Present day examples of combined cycle fuel cell systems routinely achieve at least 50% conversion efficiency. The efficiency of combined cycle fuel cell systems in converting hydrocarbon fuel into electrical energy is limited by loss mechanisms within the system that produce or lose heat and by losses of the fuel cell due to partial utilization of fuel. Typical or common attempts to improve performance or efficiency of combined cycle fuel cell systems at low fuel utilization have involved fuel and/or air-recycling. Fuel recycling in combined cycle fuel cell systems, however, requires large reformers and large high temperature blowers that are costly and technically challenging. Similarly, air recycling in combined cycle fuel cell systems requires high-temperature blowers that are not cost-effective.
In view of the foregoing, there is a need to provide cost-reduction techniques that increase the plant efficiency of combined cycle fuel cell systems through increased fuel cell efficiency that eliminate the need of fuel and/or air recycling that requires costly high temperature blowers and, potentially, heat exchangers.